With one hand (her left in the video) lift up on your partners shoulder. You will only need to lift it about an inch. You can accomplish this by just sliding your hand underneath their shoulder.
With your other hand find the muscle tissue the runs accross the upper back. Kneading the muscle tissue that is just inboard of the edge of the shoulder blade is fantastic. The shows exactly the location.
It is a difficult stroke to get right, but your partner will tell you when you have hit the spot. If you don't understand, have your partner try it on you. If they find the spot, you will know exactly why this is my personal favorite.